Sustained, intensive cognitive effort can yield substantial benefits in terms of improved decision-making and generally enhanced cognitive performance. Cognitive effort, however, is often treated as aversive, as evidenced by substantial behavioral biases against effortful engagement. Among healthy adults, there appear to be measurable differences in how aversive individuals find cognitive effort, contributing to differences in performance, unexplained by measures of intelligence. Effort may be particularly aversive in disorders of anergia and apathy. It may explain why individuals with Major Depression, for example, demonstrate average performance on low effort tasks yet perform below average on demanding tasks. Despite the import of this central bias against cognitive engagement among healthy individuals and those with mental disorder, little is understood about why cognitive effort is aversive, or about the neural systems involved in decisions about task engagement. To address this gap, this project utilizes a novel behavioral economic paradigm towards the development of a neuroeconomics of cognitive effort. The novel paradigm yields first-ever, quantitative, between and within individual estimates of the cost of cognitive effort. The first Aim of this study seeks to validate the novel paradigm by demonstrating correlations of subjective cost estimates with traditional physiological and self-report measures of cognitive effort. Next, the paradigm will be used to investigate individual and task specific factors which make cognitive effort subjectively costly. Combined fMRI and pupillometry will be used to probe specific hypotheses about physiological markers of cognitive effort. Finally, following recent advances in neuroeconomics, the novel cost estimates will be used in parametric tests of fMRI data collected as individuals decide whether to engage in demanding tasks. This approach will be used to test specific hypotheses about the neural systems involved in on-going and prospective decisions to expend cognitive effort among healthy adults. This work will also thereby provide a foundation for elucidating why cognitive effort may be particularly aversive in depression.